I haven't been through immigration at Houston, but going through it at Atlanta and Chicago recently, I'd probably want to have a couple of hours between flights. Immigration shouldn't ever take more than 30 minutes or so, but better to have a bit of cushion in case there are other delays or Customs is backed up.

It actually might be worth considering driving it (if you're already planning on a rental car) rather than flying it. The drive from Houston to Fort Worth would be around 4-5 hours, while a two-hour flight gap + the bag claim time at Love and then the hour-plus drive from Love (through downtown Dallas traffic, which depending on the time of day could stretch it into two hours) would probably get you there reasonably near the same time.